Flow reactors for chemical conversions with heterogeneous catalysts

ABSTRACT

Flow reactor having a plurality of walled conduits each having an outer surface disposed for contact with a heat-transfer medium, an inlet distribution manifold in flow communication with a downstream manifold through channels formed by heterogeneous catalytic material disposed within each conduit during operation, and a sequence of zones comprising at least two zones, the zones including the walled conduits. The walled conduits within each zone have the same or different length measured along a longitudinal coordinate of the zone, the walled conduits within each zone have essentially uniform cross-section measured in a plane perpendicular to the longitudinal coordinate thereby defining volume of the zone, and in the sequence of zones, the total cross-sectional area of the conduits in each downstream zone varies from the prior upstream zone. At least one crossover chamber is provided in flow communication with the plurality of walled conduits of a downstream zone and the plurality of walled conduits of the prior upstream zone. A shell is present for maintaining during operation the outer surface the plurality of walled conduits of each zone predominantly in contact with a heat transfer medium, the shell having an inlet in flow communication with an outlet for flow of the heat-transfer medium.

This application is a divisional of application Ser. No. 09/920,981,filed Aug. 2, 2001, now U.S. Pat. No. 7,316,804.

TECHNICAL FIELD

The present invention relates to apparatus for use in process systemswhich include exothermic chemical conversions of organic compounds tovalue added products. More particularly, the invention is flow reactorsfor exothermic chemical conversions using a fixed heterogeneous catalystwith means for control of the exotherm.

Flow reactors of the invention comprise a plurality of walled conduitseach having an outer surface disposed for contact with a heat-transfermedium, an inlet distribution manifold adapted for flow communicationwith a downstream manifold through channels formed by heterogeneouscatalytic material disposed within each conduit during operation in asequence of zones for catalyst having the same or different length alongthe longitudinal coordinate of the conduit and within each zoneessentially uniform cross-section of the conduit measured in a planeperpendicular to the longitudinal coordinate thereby defining volume ofthe zone, and the sequence of zones comprising a plurality of zones suchthat each downstream zone has a varying (i.e. larger or smaller)cross-section than the contiguous upstream zone. Reactors of theinvention generally comprise a shell adapted to maintain duringoperation the outer surface of each conduit predominantly in contactwith a heat-transfer medium, and having an inlet in flow communicationwith an outlet for the heat-transfer medium.

Another aspect of the invention includes chemical processes which usesuch flow reactors comprising a plurality of zones such that eachdownstream zone has a varying cross-section than the contiguous upstreamzone. Such processes include, for example, the continuous manufacture ofmaleic acid's intramolecular anhydride, commonly referred to as maleicanhydride wherein each downstream zone has a larger cross-section thanthe contiguous upstream zone.

BACKGROUND OF THE INVENTION

In most, if not all, processes involving chemical conversions thecontrol of temperature by means for transfer of energy is veryimportant, because chemical reactions either absorb or evolve energy.Where highly exothermic reactions are carried out in a flow reactorcontaining a fixed heterogeneous catalyst, energy evolved near theentrance of the reactants in contact with the catalyst is well-known tocause non-isothermal conditions which can result in deleteriousoverheating of the catalyst. Furthermore, non-isothermal conditions ofreaction are likely to decrease desired conversions, throughput, and/oryields of value added products.

In a large class of industrial processes the conventional design ofreaction apparatus applicable for use in carrying out highly exothermicchemical reactions uses an annular bundle of vertical contact tubeswhich are adapted to contain a fixed heterogeneous catalyst. Reactiongases are directed through the tubes containing the catalyst and theheat evolved as the reaction proceeds is removed by a heat carrier whichis circulated over the outer surface of the contact tubes.

Conventional flow reactors are illustrated in FIG. 1. Typically, suchreactors comprise a plurality of walled conduits each having an outersurface disposed for contact with a heat-transfer medium, and an inletdistribution manifold adapted for flow communication with a downstreammanifold. The conduits are of uniform cross-section throughout thelength of the reactor. In such reactors, it is sometimes difficult tobalance the heat generated during the reaction with the heat removalcapabilities of the heat transfer medium. The result is that suchreactors may operate with a “hot spot” (i.e. the location in thecatalyst bed wherein the exothermic reaction exceeds the heat removalcapabilities of the reactor), or “runaway reactions” (because ofinsufficient heat removal and often wherein oxygen is a reactant, thereactants and preferred products continue to oxidize or combust to nonproduct chemical compounds). Either of these occurrences often leads toan irreversible chemical or physical damage of the catalyst and/ordrastically reduces the life and/or performance of the catalyst.Specifically, the catalyst may melt and/or fuse together; the catalystcrystal structure or composition may be altered, any of which can causethe loss of activity and/or selectivity of the catalyst to preferredproducts.

Many designs directed to an improved heat exchange arrangement for suchreaction apparatus are known.

For example, U.S. Pat. No. 3,850,233 in the name of Oskar Wanka and JenoMihaleyi describes reaction apparatus which is a compact structure of aclosed type, without external portions, but with a complex internalarrangement including a pump which directs a flow of heat carrier mediumalong an inner tubular baffle toward the opposite end and then throughan opening in the inner tubular baffle over the contact tubes and thenback toward the pump for return through an annular space between anannular baffle and the inner tubular baffle. This complex internalarrangement is said to provide a most favorable flowing course for theheat carrier and a desirable heat exchange relationship between thedifferent media for endothermic chemical processes.

U.S. Pat. No. 3,871,445 in the name of Oskar Wanka, Friedrich Gutlhuberand Hermann Graf describes conventional design of reaction apparatus forcarrying out exothermic and endothermic chemical reactions, having ashell in which there is arranged a vertical next of contact tubes. Thesecontact tubes, which contain a catalyst material, have their oppositeends secured, in a fluid-tight manner, into respective headers and open,at their opposite ends, into upper and lower heads connected to theshell, reaction gases flowing through the contact tubes are supplied andremoved through these heads. According to the patent, a heat exchangemedium is pumped through an external heat exchanger and is supplied anddischarged to the shell through respective axially spaced annular supplyand discharge conduits, to flow over the contact tubes. Baffles arearranged in the shell to extend transversely to the length of the tubesto direct the heat exchange medium to flow alternately in opposed radialdirections over the tubes between the supply and discharge conduits. Atleast one additional annular circuit is arranged at a point of the shellintermediate between the supply and discharge conduits, is connected tothe heat exchanger and the shell, and supplies and discharges a partialamount of the heat exchange medium. In one of these complex examples,several such additional annular conduits are arranged at respectivepoints of the shell intermediate between the supply and dischargeconduits. In another, diaphragms or partitions divide the shell sideinto separate compartments each of which has a respective heat exchangerassociated therewith.

More recently, U.S. Pat. No. 3,871,445 in the name of Oskar Wanka,Friedrich Gutlhuber and Cedomil Persic describes a multistage reactionapparatus for carrying out exothermic or endothermic catalyst reactionscomprising a plurality of separate stages which are arrangedsequentially within the reaction vessel and consecutively passed throughby the reaction gas. Each stage includes a separately removable modulefilled with a catalyst, and a gas cooler in the form of a heat exchangermounted downstream of the module. Each heat exchanger represents acontrollable partial cooling circuit and all of the exchangers areinterconnected by a common circulation system serving to balance outlarger temperature variations and to supply the partial circuits. Thecommon circuit, including a man heat exchanger and a pump mounted in thereturn branch or branches of the circuit and the partial circuits orexchangers are controlled by valves or three-way control members and mayalso each comprise a pump. According to the patent such complexmultistage reaction apparatus for carrying out exothermic or endothermiccatalytic reactions in which the reaction gas subsequently passesthrough several beds of catalysts placed in transversely arranged casesand is cooled down or heated up in each such stage by means of a heatexchanger whose partial medium circuit is controllable by valves orthree-way control members and with the aid of a main circulation systemis thereby capable to hold the temperature of the reaction gas uniformlydistributed over the cross-section of the reactor and, at the entranceof the stages, on the substantially same level.

U.S. Pat. No. 4,657,741 in the name of Rudolf Vogl, describes a reactorfor carrying out exothermic and endothermic catalytic reactions whichincludes a contact tube bundle and radial admission and removal of aheat transfer medium via an annular duct for each, and a circulationthrough an external heat exchanger. Two or more circulating pumps areconnected to the annular ducts and are distributed over thecircumference. The heat exchanger can be arranged in shunt to the maincirculation and be connected with individual sections of at least oneannular duct via setting elements.

In U.S. Pat. No. 5,161,605 in the name of Friedrich Gutlhuber a tubularreactor for catalytic gas-phase reactions is described with symparallel(sic) guidance of the heat exchanger. A partial stream of theheat-exchanger medium, immediately neighboring the inlet side of thetube plate, is introduced through a by-pass channel arranged in thecenter of the bank of tubes, by-passing the bank of tubes, and at apoint downstream of the discharge area of the heat-exchanger. In thisway, according to the patent, undesirable severe local cooling in thereaction area of the bank of tubes can be avoided.

All the above-described methods are essentially based on modifying theheat transfer from the contact tubes which contain heterogeneouscatalyst after this heat has been produced by the chemical conversionreactions therein. In a paper titled, “An Alternative Method to Controlthe Longitudinal Temperature Profile in Packed Tubular Reactions (ING.CHIM. ITAL., v. 12, n. 1-2, pp. 516, gennaio-febbraio 1976) authors P.Fontana and B. Canepa credit P. H. Chalderbank, A. Caldwell and G. Rossas suggesting another method whereby the heat generation rate iscontrolled at the source, by mixing catalyst-containing pellets andinert pellets invariable ratio along the axial co-ordinate. See“Proceedings of the 45th European Symposium on Chemical ReactionEngineering” (Pergamon Press, London 1971). Charging a plurality ofcontact tubes with a mixture of catalyst-containing pellets and inertpellets according to a prescribed variable ratio along the axialco-ordinate, clearly complicates the loading process as well as recoveryof catalyst values from deactivated catalyst. Whether or not such amethod could in any way be more useful than previous described methods,it is clearly based on the regulation of the heat produced per unit oftime and volume of the bed without altering the means for transfer ofsuch heat from the outer surface of the tubes.

Authors Fontana and Canepa direct their paper to a method of obtaining apredetermined axial temperature profile by replacing the inert pelletsof Chalderbank et al, with a coaxial inert body which makes the crosssection, taken up by the active catalyst pellets, annular and variablealong the axial co-ordinate. In a theoretical example, based upon theirreduction of a mathematical model into a one-dimensional form, for anirreversible exothermic reaction of A+B going to C with B in largeexcess, a complex longitudinal profile of an axial inert body is shownas a graph. According to their mathematical analysis, the complexlongitudinal profile derived for the axial inert body should, in theoryat least, realize a constant longitudinal temperature profile. Where atypical commercial reactor for a highly exothermic conversion containsup to 20,000 or even 30,000 contact tubes which are long relative, e.g.,100 to 250 times their diameter, there remain unsolved mechanicalproblems involving fabrication and/or maintenance of a coaxial inertbody in each tube, as well as in loading catalyst into an annular spacefrom the end with smallest dimension.

Other methods of obtaining a predetermined temperature profile along theaxial co-ordinate of flow reactor containing a fixed heterogeneouscatalyst is a quench-type reactor wherein cold fluid, such as freshan/or recycled reactant, is injected into the flow at a plurality ofpoints along the axial co-ordinate or between a plurality of catalystbeds. However, in a paper titled “Technology of Lurgi's Low PressureMethanol Process” (CHEMTECH, July, 973, pp. 430-435) author E. Suppdemonstrates that for methanol production from carbon oxides andhydrogen the tubular reactor with boiling water around the tubesprovides more constant temperatures than does a quench-type reactor.Moreover, the temperature profile on the tubular reactor drops towardthe outlet and thus contributes to a better equilibrium, while eachstage of the quench-type reactor has an increasing temperature profile.

German Patent No. 29 29 300 describes a catalytic reactor, for use incarrying out endothermic or exothermic reactions, through which areactant fluid is flowed, and containing a reaction chamber filled withcatalyst material, which is in thermal contact with a heat-emitting orheat-absorbing fluid, and characterized by the fact that thecross-section surface area of the reaction chamber is varied, along withthe direction of flow of the reacting fluid, depending upon the quantityof heat required for completion of a given reaction, or the quantity ofheat released on the course of a reaction. For a proposed methanolsynthesis reactor, the diameter of the reaction chamber is varied alongthe direction of flow of the reacting fluid such that the diameter (inmm), is a constant, having a value of from 15 to 25, multiplied by thegas flow rate per reaction tube (Nm³/hr) raised to the power of aconstant having a value of from 0.12 to 0.22. As practical matter, thereaction chamber is made up of only from 2 to 5 sections of tubinghaving constant diameter.

Japanese Patent No. 61-54229 describes a chemical reactor for exothermicconversions to form methanol which reactor has of a vertical reactioncolumn filled with a granulated solid catalyst material. Gases requiredfor the reaction are introduced into the top section of the reactor, andestablishes a downward flow of reaction gas through the interior of thereactor column. Heat evolved by the reaction is removed from the columnby vaporization of water surrounding the column, which is at saturationtemperature. The reaction column consists of several sections of varyingcolumn diameter. In particular, the diameter of the upper part of thereaction column, where a relatively large amount of reaction heat isproduced, is comparatively small; while the diameter of the lowerportion of the column, where less reaction heat is liberated, is larger.

There remains, therefore, a current need for improved flow reactorapparatus for using a fixed heterogeneous catalyst which is effective inreducing the magnitude of the exotherm, reducing thermal degradation ofcatalyst activity and/or mechanical failure of catalyst/support, andthereby avoiding interruptions in service.

Advantageously, such improved flow reactor would, by means of higherselectivity and/or conversion of organic compounds, assist in improvingrecovery of value added products.

SUMMARY OF THE INVENTION

The invention is improved flow reactors for exothermic chemicalconversions using a fixed heterogeneous catalyst with means for controlof the exotherm. Apparatus of this invention is for use in a processwhich includes conversions of organic compounds to value added productsusing a selective heterogeneous catalyst.

One aspect of this invention is at least one flow reactor comprising aplurality of walled conduits each having an outer surface disposed forcontact with a heat-transfer medium, an inlet distribution manifoldadapted for flow communication with a downstream manifold throughchannels formed by heterogeneous catalytic material disposed within eachconduit during operation in a sequence of zones for catalyst having thesame or different length along the longitudinal coordinate of theconduit and within each zone essentially uniform cross-section of theconduit measured in a plane perpendicular to the longitudinal coordinatethereby defining volume of the zone, and the sequence of zonescomprising at least two zones such that each downstream zone has alarger or smaller cross-section than the contiguous upstream zone.Generally, flow reactors according to the invention, further comprise ashell adapted to maintain during operation the outer surface of eachconduit predominantly in contact with a heat-transfer medium, and havingan inlet in flow communication with an outlet for the heat-transfermedium. Preferably, the sequence of zones comprises at least threezones. Preferably, each downstream zone has a larger cross section thanthe contiguous upstream zone. Preferably, each downstream zone has alarger volume than the contiguous upstream zone.

Advantageously, in flow reactors according to invention, thecross-section of the conduit in each zone has a substantially circularform with a diameter such that the third power of the diameter is equalto the product of the volume and a geometric factor having values in arange from about 0.01 to about 0.50. Preferably the geometric factor ofeach downstream zone is larger than the contiguous upstream zone for thesequence of zones comprising at least three zones.

In once class of flow reactors according to the invention, the zones forcatalyst have a total length along the longitudinal coordinate of atleast 4 meters. Preferably in such flow reactors the cross-section ofthe conduit in each zone has a substantially circular form with adiameter such that the third power of the diameter is equal to theproduct of the volume and a geometric factor having values in a rangefrom about 0.015 to about 0.100. More preferably, the geometric factorof each downstream zone is larger than the contiguous upstream zone forthe sequence of zones comprising at least three zones.

In another class of flow reactors according to the invention, the zonesfor catalyst have a total length along the longitudinal coordinate ofless than about 3 meters. Preferably in such flow reactors thecross-section of the conduit in each zone has a substantially circularform with a diameter such that the third power of the diameter is equalto the product of the volume and a geometric factor having values in arange from about 0.10 to about 0.30. More preferably, the geometricfactor of each downstream zone is larger than the contiguous upstreamzone for the sequence of zones comprising at least three zones.

Another aspect of this invention is a flow reactor comprising:

-   -   (i) a plurality of walled conduits each having an outer surface        disposed for contact with a heat-transfer medium,    -   (ii) an inlet distribution manifold adapted for flow        communication with a downstream manifold through channels formed        by heterogeneous catalytic material disposed within each conduit        during operation,    -   (iii) a sequence of zones comprising at least two zones, said        zones comprising said walled conduits, wherein        -   (a) the walled conduits within each zone have the same or            different length measured along the longitudinal coordinate            of the zone,        -   (b) the walled conduits within each zone have essentially            uniform cross-section measured in a plane perpendicular to            the longitudinal coordinate thereby defining volume of the            zone, and        -   (c) in the sequence of zones, the total cross-sectional area            of the conduits in each downstream zone varies from the            prior upstream zone,    -   (iv) at least one crossover chamber in flow communication with        the plurality of walled conduits of a downstream zone and the        plurality of walled conduits of the prior upstream zone,    -   (v) a shell adapted to maintain during operation the outer        surface the plurality of walled conduits of each zone        predominantly in contact with a heat-transfer medium, and    -   (vi) the shell having an inlet in flow communication with an        outlet for flow of the heat-transfer medium.        Preferably, the sequence of zones comprises at least three        zones. Preferably, each downstream zone has a larger cross        section than the contiguous upstream zone. Preferably, each        downstream zone has a larger volume than the contiguous upstream        zone.

One aspect of this invention is a process which includes exothermicchemical conversions of organic compounds to value added products usinga selective heterogeneous catalyst in at least one flow reactorcomprising a plurality of walled conduits each having an outer surfacedisposed for contact with a heat-transfer medium, an inlet distributionmanifold adapted for flow communication with a downstream manifoldthrough channels formed by heterogeneous catalytic material disposedwithin each conduit during operation in a sequence of zones having thesame or different length along the longitudinal coordinate of theconduit and within each zone essentially uniform cross-section of theconduit measured in a plane perpendicular to the longitudinal coordinatethereby defining volume of the zone, and the sequence of zonescomprising at least two zones such that each downstream zone has alarger or smaller cross-section than the contiguous upstream zone.Preferably, the sequence of zones comprises at least three zones.Preferably, each downstream zone has a larger cross section than thecontiguous upstream zone. Preferably, each downstream zone has a largervolume than the contiguous upstream zone. Typically, flow reactorsaccording to the invention, further comprise a shell adapted to maintainduring operation the outer surface of each conduit predominantly incontact with a heat-transfer medium, and having an inlet in flowcommunication with an outlet for the heat-transfer medium.

A preferred class of processes of the invention include the exothermicchemical conversions of organic compounds to value added products whichcomprises oxidation of benzene or a hydrocarbon selected from the groupconsisting of n-butane, butene-1 and butadiene, to form maleicanhydride. One preferred process of the invention comprises oxidation ofn-butane to form maleic anhydride by contacting n-butane at low (lessthan lower flammability concentration of 1.6 mole percent in air)concentrations in an oxygen-containing gas with a fixed catalystcomprising principally vanadium, phosphorus and oxygen. Advantageously,the catalyst is maintained at temperatures in a range from about 360° C.to about 530° C.

In a preferred process of the invention the cross-section of eachdownstream zone is from about 5 percent to about 125 percent larger thanthe cross-section of the contiguous upstream zone. Advantageously, thevolume of each downstream zone is from about 5 percent to about 125percent larger than the volume of the contiguous upstream zone. Morepreferably the cross-section of each downstream zone which has a largercross-section than the cross-section of the contiguous upstream zone islarger by an amount such that during operation temperatures of theexotherm as measured along the centerline are no more than 50° C. higherthan the heat-transfer medium temperature.

For a more complete understanding of the present invention, referenceshould now be made to the embodiments illustrated in greater detail anddescribed below by way of examples of the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 through 5 illustrate several variations of flow reactor design.In these figures the conduits (i.e. reactor tubes) connecting the inletdistribution manifold with the outlet manifold and typically packed withcatalyst are represented by the shaded areas on the figures. Theunshaded area surrounding the reactor tubes represent the area occupiedby the heat transfer medium.

FIG. 1 illustrates a conventional prior art multi-tubular flow reactor,wherein the conduits are of constant diameter.

FIG. 2 illustrates a multi-tubular flow reactor, wherein each conduit iscomprised of a plurality of zones and wherein each zone from inlet tooutlet is of increasing cross-sectional area.

FIG. 3 illustrates a multi-tubular flow reactor, wherein each conduit isof continuously increasing cross-sectional area from inlet to outlet.

FIG. 4 illustrates a multi-tubular flow reactor, comprised of aplurality of zones wherein (i) all conduits in single zone are ofconstant cross sectional area, (ii) each zone is connected to thesubsequent zone by a crossover chamber which will collect the flow fromthe previous zone and redistribute the flow to the conduits of the nextzone, and (iii) the cross sectional area of each conduit in the zonevaries from one zone to the next.

FIG. 5 illustrates a multi-tubular flow reactor, comprised of aplurality of zones wherein (i) the conduits are of constant crosssectional area in all zones, (ii) each zone is connected to thesubsequent zone by a crossover chamber which will collect the flow fromthe previous zone and redistribute the flow to the conduits of the nextzone, and (iii) the number of conduits in each zone varies such that thetotal cross-sectional area of all conduits in each zone varies from onezone to the next.

It will be understood by those skilled in the art that, as the drawingis diagrammatic, further items of equipment such as condensers, heatexchangers, reflux drums, column reboilers, pumps, vacuum pumps,temperature sensors, pressure sensors, pressure relief valves, controlvalves, flow controllers, level controllers, holding tanks, storagetanks, and the like, would additionally be required in a commercialplant. The provision of such additional items of equipment forms no partof the present invention and is in accordance with conventional chemicalengineering practice.

The instant invention relates to a catalytic reactor, through which oneor more reactants are flowed while undergoing exothermic reactions andconversion to at least one product, the catalytic reactor comprises aplurality of walled conduits each having an outer surface disposed forcontact with a heat-transfer medium, and an inlet distribution manifoldadapted for flow communication with a downstream manifold. Disposedwithin each conduit is a heterogeneous catalytic material which is incontact with the flow of reactants and reaction products duringoperation. Each conduit comprises a sequence of zones having the same ordifferent length along the longitudinal coordinate of the conduit. Eachzone is of essentially uniform cross-section of the conduit measured ina plane perpendicular to the longitudinal coordinate thereby definingvolume of the zone. In the sequence of zones, each downstream zone has adifferent cross-section than the contiguous upstream zone. Typically,such reactors comprise at least two zones, preferably three zones.Preferably, the cross section of each zone is larger than the contiguousupstream zone. Preferably, each downstream zone has a larger volume thanthe contiguous upstream zone. Generally, flow reactors according to theinvention, further comprise a shell adapted to maintain during operationthe outer surface of each conduit predominantly in contact with theheat-transfer medium, and having an inlet in flow communication with anoutlet for the heat-transfer medium.

One embodiment of the invention is illustrated in FIG. 2. The reactantsand any diluents are fed via one or more feed lines 1 into the inletdistribution manifold 2. The reactants and diluents pass through aplurality of conduits 3 to a downstream collection manifold 4. Eachconduit comprises zones 5, 6, 7 and 8 of varying cross-section andlength. The heat transfer medium (HTM) enters annular space 9 (i.e. theunshaded area) surrounding the conduit via line 10 and exits via line11. The reactor effluent exits via line 12. In this figure, the conduitsconnecting the inlet distribution manifold with the outlet manifoldcontain catalyst and are represented by the shaded areas in the figure.

Another embodiment of this invention is illustrated in FIG. 3 whereinthe conduits 3 are of gradually increasing cross section measured in aplane perpendicular to the longitudinal coordinate, as opposed to thestep change in cross section from the prior zone shown in FIG. 2. Thegradually increasing cross section of the conduits in FIG. 2 isessentially equivalent to an infinite number of zones.

Yet, another embodiment of this invention is a flow reactor comprising:

-   -   (i) a plurality of walled conduits each having an outer surface        disposed for contact with a heat-transfer medium,    -   (ii) an inlet distribution manifold adapted for flow        communication with a downstream manifold through channels formed        by heterogeneous catalytic material disposed within each conduit        during operation,    -   (iii) a sequence of zones comprising at least two zones, said        zones comprising said walled conduits, wherein        -   (a) the walled conduits within each zone have the same or            different length measured along the longitudinal coordinate            of the zone,        -   (b) the walled conduits within each zone have essentially            uniform cross-section measured in a plane perpendicular to            the longitudinal coordinate thereby defining volume of the            zone, and        -   (c) in the sequence of zones, the total cross-sectional area            of the conduits in each downstream zone varies from the            prior upstream zone,    -   (iv) at least one crossover chamber in flow communication with        the plurality of walled conduits of a downstream zone and the        plurality of walled conduits of the prior upstream zone,    -   (v) a shell adapted to maintain during operation the outer        surface the plurality of walled conduits of each zone        predominantly in contact with a heat-transfer medium, and    -   (vi) the shell having an inlet in flow communication with an        outlet for flow of the heat-transfer medium.        This embodiment of the invention is illustrated in FIGS. 4        and 5. The reactants and any diluents are fed via one or more        feed lines 1 into the inlet distribution manual 2. The reactants        and diluents pass through a plurality of catalyst containing        conduits 3 in a first zone to a first crossover chamber 4. The        reactants, any diluents and any reaction products from the        previous zone flow pass from crossover chamber 4 through a        plurality of catalyst containing conduits 5 in a second zone to        a second crossover chamber 6. The reactants, any diluents and        any reaction products from second zone pass from crossover        chamber 6 through a plurality of catalyst containing conduits 7        in a third zone to a third crossover chamber 8. The reactants,        any diluents and any reaction products from third zone pass from        crossover chamber 8 through a plurality of catalyst containing        conduits 9 in a fourth zone to an outlet manifold 10. The        reactor effluent exits the reactor via line 11. The heat        transfer medium enters annular space 12 (i.e. the unshaded)        surrounding the conduits in each zone via line 13 and exits via        line 14. Optionally, additional reactants and/or diluents may be        introduced into one or more of the crossover chambers via lines        15, 16 or 17. In these figures, the catalyst containing conduits        are represented by the shaded areas on the figures.

In FIG. 4, each zone contains an equal or different number of conduitswith the cross section of each conduit increasing in each subsequentzone. In FIG. 5, the cross section of each individual conduit in allzones is constant, but the number of conduits varies in each zone suchthat the total cross sectional area of the conduits in each downstreamzone increases from the prior upstream zone.

This latter embodiment of the invention may additionally be conducted inone or more reaction vessels connected in series, such that eachreaction vessel comprises one or more zones. In this configuration, thecrossover chamber comprises the collection and distribution manifolds atthe bottom and top, respectively, of contiguous reaction vessels.Further, the reaction vessels could be operated with individual orcommon cooling loops (i.e. the flow path of the heat transfer medium).An “individual cooling loop” comprises a flow path for the heat transfermedium which passes through only one reaction vessel. In contrast, a“common cooling loop” comprises a flow path for the heat transfermedium, which passes through more than one reaction vessel.

A key feature in the design and utilization of the flow reactorsdescribed herein is to control the residence time of the reactants ineach zone by varying both the cross sectional area and the length ofeach zone (thereby varying the volume of each zone) such that the heatgenerated during the exothermic reaction(s) occurring in the conduits ineach zone does not exceed the amount of heat capable of beingtransferred to and removed by the heat transfer medium in the annularspace surrounding the conduits. Flow reactors designed and operated inthis fashion essentially “control” the extent of the reaction occurringin each zone to the extent that the formation of “hot spots”, “runawayreactions”, and/or reactor instability can be avoided.

As illustrated in the Figures, the total cross section of the conduitsincreases from the prior upstream zone. However, this invention alsocontemplates reactor designs wherein the total cross section of theconduits in a given zone decreases from the prior upstream zone.

Advantageously, in flow reactors according to invention, thecross-section of the conduit in each zone has a substantially circularform with a diameter such that the third power of the diameter is equalto the product of the volume and a geometric factor having values in arange from about 0.01 to about 0.50. Preferably the geometric factor ofeach downstream zone is larger than the contiguous upstream zone for thesequence of zones comprising at least three zones.

In one class of flow reactors according to the invention, the zones forcatalyst have a total length along the longitudinal coordinate of atleast 4 meters. Preferably in such flow reactors the cross-section ofthe conduit in each zone has a substantially circular form with adiameter such that the third power of the diameter is equal to theproduct of the volume and a geometric factor having values in a rangefrom about 0.015 to about 0.100. More preferably, the geometric factorof each downstream zone is larger than the contiguous upstream zone forthe sequence of zones comprising at least three zones.

In another class of flow reactors according to the invention, the zonesfor catalyst have a total length along the longitudinal coordinate ofless than about 3 meters. Preferably in such flow reactors thecross-section of the conduit in each zone has a substantially circularform with a diameter such that the third power of the diameter is equalto the product of the volume and a geometric factor having values in arange from about 0.10 to about 0.30. More preferably, the geometricfactor of each downstream zone is larger than the contiguous upstreamzone for the sequence of zones comprising at least three zones.

Generally in multi-tubular fixed-bed reactors, reacting gas pass throughthe tubes, while a suitable heat transfer medium or coolant on theoutside of the tubes removes the heat of reaction. Tube diameter iscommonly from about 3 to about 5 centimeters; because greater diametersmight give insufficient area for heat removal and lead to excessive hotspot temperatures in the center of the tube. Typical tube lengths rangeup to about 20 meters, however tube length may be limited to about 15meters or less; because longer tubes may give unacceptably high-pressuredrop.

With all multi-tubular fixed bed reactors, uniform flow through thetubes is important in achieving optimum performance. Uniform flow isespecially important for exothermic reactors. If flow is slower throughsome tubes, heat removal is impaired, and local temperature spikes canlead to excessive production of undesired side products, e.g. oxides ofcarbon, in those tubes. If flow through some of the tubes is faster thanaverage, the reduced residence time leads to lower conversions ofreactants in those tubes. Flow rate depends on the resistance in thetubes, which is determined largely by the packed density of thecatalyst. Procedures have been developed to load the catalyst pelletsuniformly, with minimum breakage and dust formation. Some operators usepre-weighed bags of catalyst that they empty one by one into the tubes.Regardless of the loading method, the flow through each tube should begauged by measuring the pressure drop under standard conditions. Tubeswhose pressure drop is statistically lower or higher than the averageshould be emptied and refilled.

This invention also comprises a process for oxidizing benzene orhydrocarbons containing four carbon atoms such as n-butane, butene-1,and butadiene to maleic anhydride using at last one oxidation reactorcomprising a plurality of walled conduits according to the invention.Maleic acid, cis-butenedioic acid, and fumaric acid, trans-butenedioicacid are important examples of unsaturated dicarboxylic acids.Preferably, maleic anhydride is produced by catalytic oxidation ofn-butane according to the simple chemical equation:CH₃CH₂CH₂CH₃+3.5O₂=>C₄H₂O₃+4H₂OThe main side reaction is the oxidation of n-butane to carbon oxides:CH₃CH₂CH₂CH₃+5.5O₂=>2CO₂+2CO+5H₂OWhile the ratio of carbon monoxide to carbon dioxide is shown as 1:1,obviously the ratio depends on catalyst and conditions of reaction. Bothreactions are well known to be highly exothermic.

Catalyst selection depends somewhat on the particular hydrocarbon feed.For a feed of benzene, a catalyst comprising chiefly of molybdenum,vanadium and oxygen is preferred for best results, but for n-butane, acatalyst comprising mainly of phosphorus, vanadium, and oxygen, ispreferred for best results. With a feed of unsaturated hydrocarboncontaining about four carbon atoms, a catalyst comprising principallytungsten, phosphorus and oxygen is likely to show good results.

Generally contacting the hydrocarbon in the presence of oxygen with thecatalyst is conducted at temperatures in a range from about 360° C. toabout 530° C., but preferably not over about 450° C. The oxidation ofn-butane to form maleic anhydride may be accomplished by contactingn-butane in low concentration in oxygen with the described catalyst. Airis entirely satisfactory as a source of oxygen, but synthetic mixturesof oxygen and diluent gases such as nitrogen may also be employed. Airenriched with oxygen may be used.

The gaseous feed stream to the oxidation reactors will normally containair and about 0.2 to about 1.7 mole percent of the hydrocarbon such asbenzene, butane, butene, or butadiene. About 0.8 to about 1.5 molepercent of the hydrocarbon is satisfactory for optimum yield of maleicanhydride for the process of this invention. Although higherconcentrations may be employed, explosive hazards may be encountered.Lower concentrations of the hydrocarbon feedstock, less than about onepercent, of course, will reduce the total yield obtained at equivalentflow rates and, thus, are not normally employed for economic reasons.The flow rate of the gaseous stream through the reactor may be variedwithin rather wide limits, but the preferred range of operations is atthe rate of about 100 to about 4000 cc of feed per cc of catalyst perhour and more preferably about 1000 to about 2400 cc of feed per cc ofcatalyst per hour. Lower flow rates make the butane oxidation processuneconomical. A catalyst should be effective at flow rates of about 1200to about 2400 cc of hydrocarbon feed per cc of catalyst per hour. Thereare catalysts which show good promise but when subjected to the hourlyspace velocity designated above show very poor yields. The amount ofwater added is about 1000 to about 40,000 parts per million by weight ofthe reactor feed gas stream. The preferred amount of water added isabout 5000 to about 35,000 parts per million by weight of the reactorfeed gas stream. Residence times of the gas stream will normally be lessthan about four seconds, more preferably less than about one second, anddown to a rate where less efficient operations are obtained. The flowrates and residence times are calculated at standard conditions of 760mm of mercury and at 0° C.

A variety of reactors will be found to be useful and multiple tube heatexchanger-type reactors are quite satisfactory. The tubes of suchreactors may vary in diameter from about one-quarter inch to about threeinches, and the length may be varied from about three to about ten ormore feet. The oxidation reaction is an exothermic reaction and,therefore, relatively close control of the reaction temperatures shouldbe maintained. It is desirable to have the surface of the reactors at arelatively constant temperature and some medium is needed to conductheat from the reactors, such as lead and the like, but it has been foundthat eutectic salt baths are completely satisfactory. One such salt bathis a sodium nitrate, sodium nitrite, and potassium nitrate eutecticconstant temperature mixture. An additional method of temperaturecontrol is to use a metal block reactor whereby the metals surroundingthe tube act as a temperature regulating body. As will be recognized byone skilled in the art, the heat exchanger medium may be kept at theproper temperature by heat exchangers and the like. The reactor orreaction tubes may be iron, stainless steel, carbon steel, nickel, glasstubes such as Vycor, and the like. Both carbon steel and nickel tubeshave excellent long life under the conditions of the reaction describedherein. Normally, the reactors contain a preheat zone containing aninert material such as one-quarter inch Al₂O₃ (Alundum™) pellets, inertceramic balls, nickel balls, or chips and the like present at aboutone-half to one-tenth the volume of the active catalyst present.

The temperature of reaction may be varied within some limits, butnormally the reaction should be conducted at a temperature within arather critical range. The oxidation reaction is exothermic and oncereaction is underway, the main purpose of the salt bath or other mediumis to conduct heat away from the walls of the reactor and control thereaction. Better operations are normally obtained when the reactiontemperature employed is no greater than about 10 to about 30° C. abovethe salt bath temperature. The temperature of the reactor, of course,will also depend to some extent upon the size of the reactor andhydrocarbon feedstock concentration.

The reaction may be conducted at atmospheric, super-atmospheric or belowatmospheric pressure. Typically, the reaction is conducted atsuperatmospheric pressure so that the exit pressure will be at leastslightly higher than the ambient pressure to ensure a positive flow fromthe reactor. The pressure of the inert gases must be sufficiently higherto overcome the pressure drop through the reactor.

Maleic anhydride may be recovered by a number of ways well known tothose skilled in the art. For example, the recovery may be by directcondensation or by adsorption in suitable media, with specific operationand purification of the maleic anhydride.

The following examples will serve to provide a fuller understanding ofthe invention, but it is to be understood that these examples are givenfor illustrative purposes only and should not be interpreted as limitingthe invention in any way.

This invention includes processes for oxidation of o-xylene to phthalicanhydride using at least one oxidation reactor comprising a plurality ofwalled conduits according to the invention. Preferably, phthalicanhydride is produced by catalytic oxidation of o-xylene at from about350° C. to about 400° C. with a solid catalyst, such as avanadium-titanium oxygen catalyst according to the simple chemicalequation:1,2−(CH₃)₂C₆H₄+3O₂=>C₆H₄=1,2−(CO)₂O+3H₂OThe main side reaction is the complete oxidation of o-xylene. Theseoxidation reactions are well known to be highly exothermic.

This invention also comprises a process for oxidation of ethylene toethylene oxide using at least one oxidation reactor comprising aplurality of walled conduits according to the invention. Preferably,ethylene oxide is produced by catalytic oxidation of ethylene withsilver supported on a silica carrier along with some aluminum oxideaccording to the simple chemical equation:2CH₂CH₂+O₂=>2O(CH₂)₂The main side reaction is the complete oxidation of ethylene:CH₂CH₂+3O₂=>2CO₂+2H₂OBoth reactions are well known to be highly exothermic.

While the conditions for oxidation of ethylene to ethylene oxide includetemperatures not exceeding about 250° C. the heat of reaction is usuallyremoved by boiling water on the outside of the tubes. At temperatures ofreaction exceeding about 250° C., the liquid coolant boiling on theoutside of the tubes is a hydrocarbon.

As it forms, the vapor leaves the reactor shell and is externallycondensed (thus generating steam). Liquid coolant is returned to thereactor shell, preferably by gravity, to avoid reliance on a pump thatcould fail. The main advantage of using a refinery fraction boiling inthe kerosene range is that the pressure on the shell side of the reactoris less than about 3 bar. The disadvantage is that the coolant isflammable.

The temperature in the reactor tubes is maintained at the desired levelby automatically adjusting the pressure of the boiling coolant on theoutside of the tubes. Turbulent mixing of the coolant is required tominimize radial variations in the reactor temperature and to provide ahigh heat-transfer coefficient. However, the temperature is not uniformalong the length of the tubes: Near the top of the tubes, thetemperature rises rapidly (the “hot spot”), then drops again to a nearlyuniform temperature for the last 60-65 percent of the tube length.

Avoiding a temperature that is too high at the hot spot is important;high temperatures cause lower selectivity to ethylene oxide because moreof the ethylene is burned to carbon dioxide and water. Moreover, theexothermic heat of reaction is much higher for carbon dioxide formation,and can exceed the local heat removal capacity of the coolant (a“runaway reaction”). This accelerating reaction must be avoided.

Reactor pressure can range from about 1 to about 30 bar, but istypically about 15 to about 25 bar. The pressure is chosen with regardto safety, handling, equipment, and other practical considerations.Higher pressures permit the use of smaller equipment to handle a givenproduction rate. However, the flammable limit envelope expands somewhatat higher pressures, thereby reducing design options for selecting anoperating point that is sufficiently removed from the unsafe operatingregion.

Typically the gas hourly space velocity is in a range of about 2500 toabout 7000 hr⁻¹. Gas hourly space velocity (GHSV) is the volume of gas(measured at standard conditions) that passes through each volume ofcatalyst bed per hour.

This invention also comprises a process for preparation of acrylic acidby oxidation of propylene using at least one oxidation reactorcomprising a plurality of walled conduits according to the invention.Typically, the catalytic vapor phase oxidation of propylene is carriedout in two stages, i.e., oxidation of propylene to acrolein and acrylicacid, and oxidation of acrolein to acrylic acid.

In the first stage of catalytic oxidation, propylene is reactedaccording to the simple chemical equipments:CH₂CHCH₃+O₂=>CH₂CHCOH+H₂Oand2CH₂CHCH₃+3O₂=>2CH₂CHCOOH+2H₂OThe main side reactions are oxidation of propylene to carbon oxides:2CH₂CHCH₃+7.5O₂=>3CO+3CO₂+6H₂Oand formation of acetic acid:CH₂CHCH₃+2.5O₂=>CH₃COOH+CO₂+H₂OThe main reaction in the second oxidation, acrolein to acrylic acid, isshown below:2CH₂CHCOH+5.5O₂=>3CO+3CO₂+4H₂OThese oxidation reactions are well known to be highly exothermic.

Generally, processes of the invention are divided into an oxidationsection and a purification section. In the oxidation section, propyleneis catalytically oxidized with air to acrolein (mainly) in the firststage, and to acrylic acid by further air oxidation of the intermediatemixture in the second stage. One or both oxidation reactions are carriedout in at least one fixed bed oxidation reactor comprising a pluralityof walled conduits according to the invention. In the purificationsection, the gaseous reactor effluent is cooled by direct contact withwater, and acrylic acid is absorbed by the water. The aqueous acrylicacid solution azeotropically distilled with, for example, methylisobutyl ketone, which forms an azeotrope with water and thus removesthe water from the product stream. Beneficially, the crude acrylic acidis purified in a continuous two-column system to remove the light andheavy ends.

According to one embodiment of the invention liquid propylene (95percent by weight propylene and 5 percent by weight propane), storedunder pressure, is vaporized and mixed with compressed hot air.Advantageously, steam at elevated pressure up to about 250 psia is usedas a diluent in the feed for better reactor operation. The feed mixturemay be preheated before it enters one or more first stage oxidationreactor. The tubes are packed with a solid oxidation catalyst, forexample a molybdenum-bismuth-tungsten mixed oxide catalyst. Molten salton the shell side acts as a coolant, which is used to generate mediumpressure steam in salt bath coolers. Typically, the pressure drop acrossthe reactor tubes is less than about 20 psi. preferably less than about10 psi. Effluents from each of the first-stage oxidation reactors aremixed with additional compressed hot air and medium pressure steam. Themixture is preheated to the reaction temperature before it enters thesecond-stage oxidation reactors, where the acrolein is oxidized toacrylic acid.

Preferably, the second-stage oxidation reactors are also shell-and-tubedesign comprising a plurality of walled conduits according to theinvention, with a molybdenum-vanadium mixed oxide catalyst in the tubesand molten salt in the shell. Again the molten salt is used to generatemedium pressure steam in salt bath coolers. Typically, the pressure dropacross the reactor tubes is less than about 20 psi. preferably less thanabout 10 psi.

The gaseous product streams from each of the second-stage oxidationreactors are fed to a quench absorber near the bottom of the absorber.Water is used to quench the product mixture and absorb the acrylic acid,unreacted acrolein, acetic acid, and other nonvolatile by-products.Gases such as CO, CO2, O2, N2, steam, propane, and unreacted propyleneexit the top of the absorber.

The gaseous acrylic acid solution leaving the bottom of the quenchabsorber is cooled and partially recycled to the top of the column asthe liquid absorbent. A polymerization inhibitor, such as hydroquinone,is added to the system through the recycled absorbent stream. Theaqueous product, which typically contains about 40 percent by weight ofacrylic acid enters the purification section of the process as feed tothe azeotropic distillation column.

In another embodiment, this invention includes processes for theoxyacetylation of ethylene to produce vinyl acetate monomer using atleast one oxidation reactor comprising a plurality of walled conduitsaccording to the invention. Preferably, vinyl acetate monomer isproduced by catalytic oxyacetylation of ethylene at temperatures of fromabout 150° C. to about 200° C. with a solid catalyst, such as a silicasupport impregnated with palladium, gold and potassium acetate catalystaccording to the simple chemical equation:2(CH₂)₂+2CH₃COOH+O₂=>2CH₃COOCH+2H₂OThe main side reaction is the complete oxidation of reactants. Theseoxidation reactions are well known to be highly exothermic.

In yet another embodiment, this invention includes processes for theoxychlorination of ethylene to produce ethylene dichloride(1,2-dichloroethane) using at least one oxidation reactor comprising aplurality of walled conduits according to the invention. Preferably,ethylene dichloride is produced by catalytic oxychlorination of ethylenein the gas phase at from about 200° C. to about 250° C. with a solidcatalyst, for example, cupric chloride with or without other activeingredients, impregnated on a porous support such as alumina,alumina-silica, or diatomaceous earth, preferably alumina, according tothe simple chemical equation:2(CH₂)₂+4HCl+O₂=>2CH₂Cl−CH₂Cl+2H₂OAmong the additives, alkaline metal chlorides are most useful preferablychlorides of potassium, lithium or sodium (KCl, LiCl, or NaCl). The mainside reaction is the complete oxidation of reactants. These oxidationreactions are well known to be highly exothermic.

In view of the features and advantages of the method and apparatus forexothermic chemical conversions of organic compounds to value addedproducts in accordance with this invention as compared to other flowreactors previously proposed and/or employed for control of the exothermin vapor-phase processes using a fixed heterogeneous catalyst, thefollowing examples are given.

EXAMPLES

The test program evaluated each reactor tube configuration for thehighly exothermic chemical conversions in the making of maleic anhydrideby oxidation of n-butane. In the example, the terms “conversion”,“selectivity”, and “yield” are defined as follows:

${{Conversion}\mspace{14mu}(\%)} = {\frac{{Moles}\mspace{14mu} n\text{-}{butane}\mspace{14mu}{consumed}}{{Moles}\mspace{14mu} n\text{-}{butane}\mspace{14mu}{in}\mspace{14mu}{feed}} \times 100}$${{Selectivity}\mspace{14mu}(\%)} = {\frac{{Moles}\mspace{14mu}{maleic}\mspace{14mu}{anhydride}\mspace{14mu}{produced}}{{Moles}\mspace{14mu} n\text{-}{butane}\mspace{14mu}{consumed}} \times 100}$Yield  (Wt.  %) = [Conversion  (%)] × [Selectivity  (%)] × 169The following demonstrations employed a pilot-scale flow reactorcontaining a fixed heterogeneous catalyst for continuous vapor-phaseoxidation of n-butane with air. The pilot-scale system included a48-inch reactor tube which was immersed in a heat transfer mediumcomprising of molten salt. The reactor tube was equipped with a ⅛-inchaxial thermowell which allowed determination of the temperature profileof the catalyst bed along the axial co-ordinate. In each example asuitable amount of fresh commercial catalyst, i.e., avanadium-phosphorus-molybdenum-oxygen catalyst, was charged to thereactor tube.

Comparative Example A

In this example the test program described above was used to evaluate acylindrical reference reactor having a uniform cross-section of 5.07 cm²as measured in a plane perpendicular to the centerline and a catalystzone volume of about 618 cm³. Concentration of butane in the feed was1.5 mole percent. At 2000 VHSV in the catalyst zone and a molten salttemperature of 419° C., the maleic anhydride yield was 65 percent byweight after the catalyst was fully activated. The temperature profileof the catalyst bed along the axial co-ordinate exhibited a maximum of471° C.

Example 1

In this example the test program described above was used to demonstratea reactor having a volume for catalyst of about 613 cm³ distributedaccording to the invention into four contiguous cylindrical zones asshown in Table 1.

TABLE 1 Percent of Catalyst Tube Tube Fraction of Total CatalystGeometric Zone Length I.D. Axial Length Volume Factor × 10² I 14 in.0.81 in. 0.2917 19.28 7.37 II 12 in. 0.87 in. 0.2500 19.07 9.23 III 12in. 0.98 in. 0.2500 24.29 10.42 IV 10 in. 1.33 in. 0.2083 37.36 16.99Concentration of butane in the feed was 1.5 mole percent. At 2000 VHSVin the catalyst zone and a molten salt temperature of 395° C., themaleic anhydride yield was 91 percent by weight after the catalyst wasfully activated. The temperature profile of the catalyst bed exhibited amaximum of 446° C.

Example 2

In this example the test program described above was used to demonstratea reactor having a volume for catalyst of about 620 cm³ distributedaccording to the invention into four contiguous cylindrical zones asshown in Table II.

TABLE II Percent of Catalyst Tube Tube Fraction of Total CatalystGeometric Zone Length I.D. Axial Length Volume Factor × 10² I 14 in.0.81 in. 0.2917 19.28 7.37 II 14 in. 0.87 in. 0.2917 22.01 7.91 III 14in. 1.12 in. 0.2917 36.74 10.22 IV  6 in. 1.33 in. 0.1250 22.18 28.31Concentration of butane in the feed was 1.5 mole-percent. At 2000 VHSVin the catalyst zone and a molten salt temperature of 389° C., themaleic anhydride yield was 90 percent by weight after the catalyst wasfully activated. The temperature profile of the catalyst bed exhibited amaximum of 426° C.

Example 3

In this example the test program described above was used to demonstratea reactor having a volume for catalyst of about 616 cm³ distributedaccording to the invention into four contiguous cylindrical zones asshown in Table III.

TABLE III Percent of Catalyst Tube Tube Fraction of Total CatalystGeometric Zone Length I.D. Axial Length Volume Factor × 10² I 12 in.0.74 in. 0.2500 14.64 7.91 II 12 in. 0.87 in. 0.2500 18.98 9.23 III 13in. 1.12 in. 0.2708 26.19 9.62 IV 11 in. 1.33 in. 0.2292 40.19 15.44Concentration of butane in the feed was 1.5 mole percent. At 2000 VHSVin the catalyst zone and a molten salt temperature of 384° C. The maleicanhydride yield was 89 percent by weight after the catalyst was fullyactivated. The temperature profile of the catalyst bed exhibited amaximum of 455° C.

Example 4

In this example the test program described above was used to demonstratea reactor having a volume for catalyst of about 620 cm³ distributedaccording to the invention into four contiguous cylindrical zones asshown in Table IV.

TABLE IV Percent of Catalyst Tube Tube Fraction of Total CatalystGeometric Zone Length I.D. Axial Length Volume Factor × 10² I 14 in.0.81 in. 0.2917 19.08 7.37 II 14 in. 0.87 in. 0.2917 22.01 7.91 III 14in. 1.12 in. 0.2916 36.74 10.22 IV 14 in. 1.33 in. 0.1250 22.18 28.31Concentration of butane in the feed was 2.0 mole percent. At 2000 VHSVin the catalyst zone and a molten salt temperature of 383° C., themaleic anhydride yield was 81 percent by weight after the catalyst wasfully activated. The temperature profile of the catalyst bed exhibited amaximum of 446° C.

Example 5

In this example the test program described above was used to demonstratea reactor having a volume for catalyst of about 617 cm³ distributedaccording to the invention into four contiguous cylindrical zones asshown in Table V.

TABLE V Percent of Catalyst Tube Tube Fraction of Total CatalystGeometric Zone Length I.D. Axial Length Volume Factor × 10² I  6 in.0.98 in. 0.1250 12.07 20.83 II 15 in. 0.87 in. 0.3125 23.67 7.39 III 11in. 0.98 in. 0.2292 22.11 11.37 IV 16 in. 1.12 in. 0.3333 42.15 8.94Concentration of butane in the feed was 1.5 mole percent. At 2000 VHSVin the catalyst zone and a molten salt temperature of 384° C., themaleic anhydride yield was 79 percent by weight after the catalyst wasfully activated. The temperature profile of the catalyst bed exhibited amaximum of 471° C.

The following demonstrations employed a mock-up of commercial flowreactor containing a fixed heterogeneous catalyst for continuousvapor-phase oxidation of n-butane with air. The mock-up system includeda 4.6-meter reactor tube which was immersed in a heat transfer mediumcomprising of molten salt. The reactor tube was equipped with an axialthermowell which allowed determination of the temperature profile of thecatalyst bed along the axial co-ordinate. In each example a suitableamount of fresh commercial catalyst, i.e., a vanadium-phosphorus oxygencatalyst, was charged to the reactor tube.

Comparative Example B

In this example the test program described above was used to evaluate acylindrical reference reactor having a uniform cross-section in a planeperpendicular to the centerline and a catalyst zone of about 2,012 cm³.Concentration of butane in the feed was 1.7 mole percent. At 2000 VHSVin the catalyst zone and a molten salt temperature of 413° C., themaleic anhydride yield was 77 percent by weight after the catalyst wasfully activated. The temperature profile of the catalyst bed along theaxial co-ordinate exhibited a maximum of 491° C.

Example 6

In this example the test program described above was used to demonstratea reactor having a volume for catalyst of about 617 cm³ distributedaccording to the invention into four contiguous cylindrical zones asshown in Table V.

TABLE VI Percent of Catalyst Tube Tube Fraction of Total CatalystGeometric Zone Length I.D. Axial Length Volume Factor × 10² I 45 in.0.81 in. 0.2486 15.75 2.69 II 56 in. 0.87 in. 0.3094 22.02 2.27 III 56in. 1.12 in. 0.3094 38.16 2.77 IV 24 in. 1.33 in. 0.1326 24.07 7.49Concentration of butane in the feed was 1.7 mole percent. At 2000 VHSVin the catalyst zone and a molten salt temperature of 396° C., themaleic anhydride yield was 96 percent by weight after the catalyst wasfully activated. The temperature profile of the catalyst bed exhibited amaximum of 429° C.

For the purposes of the present invention, “predominantly” is defined asmore than about fifty percent. “Substantially” is defined as occurringwith sufficient frequency or being present in such proportions as tomeasurably affect macroscopic properties of an associated compound orsystem. Where the frequency or proportions for such impact is not clear,substantially is to be regarded as about ten percent or more.“Essentially” is defined as absolutely except that small variationswhich have no more than a negligible effect on macroscopic propertiesand final outcome are permitted, typically up to about one percent.

1. A flow reactor comprising: (i) a plurality of walled conduits eachhaving an outer surface disposed for contact with a heat-transfermedium, (ii) an inlet distribution manifold in flow communication with adownstream manifold through channels formed by heterogeneous catalyticmaterial disposed within each conduit during operation, (iii) a sequenceof zones comprising at least two zones, said zones comprising saidwalled conduits, wherein (a) the walled conduits within each zone havethe same or different length measured along a longitudinal coordinate ofthe zone, (b) the walled conduits within each zone have essentiallyuniform cross-section measured in a plane perpendicular to thelongitudinal coordinate thereby defining volume of the zone, and (c) inthe sequence of zones, the total cross-sectional area of the conduits ineach downstream zone varies from the prior upstream zone, (iv) at leastone crossover chamber in flow communication with the plurality of walledconduits of a downstream zone and the plurality of walled conduits ofthe prior upstream zone, (v) a shell for maintaining during operationthe outer surface of the plurality of walled conduits of each zonepredominantly in contact with a heat transfer medium, and (vi) the shellhaving an inlet in flow communication with an outlet for flow of theheat-transfer medium.
 2. The flow reactor of claim 1, wherein the crosssection and length of each zone are sized so that the heat generatedduring any exothermic reactions occurring inside the conduits of thezone does not exceed the amount of heat capable of being transferred toand removed by the heat transfer medium surrounding the conduits.
 3. Theflow reactor according to claim 1, wherein the sequence of zonescomprises at least three zones.
 4. The flow reactor according to claim1, wherein that each downstream zone has a larger cross-section than thecontiguous upstream zone.
 5. The flow reactor according to claim 1,wherein each downstream zone has a larger volume than the contiguousupstream zone.
 6. The flow reactor according to claim 1, wherein atleast one crossover chamber has an inlet in flow communication with theplurality of walled conduits of a downstream zone.
 7. The flow reactoraccording to claim 1, further comprising a shell for maintaining duringoperation the outer surface of each conduit predominantly in contactwith a heat-transfer medium, and having an inlet in flow communicationwith an outlet for the heat-transfer medium.
 8. The flow reactoraccording to claim 7, wherein cross section of the conduit in each zonehas a substantially circular form with a diameter such that the thirdpower of the diameter is equal to the product of the volume and ageometric factor having values in a range from about 0.01 to about 0.50.9. The flow reactor according to claim 7, wherein the geometric factorof each downstream zone is larger than the contiguous upstream zone forthe sequence of zones comprising at least three zones.
 10. The flowreactor according to claim 1, wherein the zones for catalyst have atotal length along the longitudinal coordinate of at least 4 meters. 11.The flow reactor according to claim 1, wherein the cross-section of theconduit in each zone has a substantially circular form with a diametersuch that the third power of the diameter is equal to the product of thevolume and a geometric factor having values in a range from about 0.015to about 0.100.
 12. The flow reactor according to claim 11, wherein thegeometric factor of each downstream zone is larger than the contiguousupstream zone for the sequence of zones comprising at least three zones.